The present invention relates to alternating current power supplies, and more particularly to an adjustable voltage power supply suitable for supplying constant power to a variable resistance load.
Many industrial loads have resistances which vary with temperature or with time. For example, glass melting furnaces, where the glass is heated by electric current, have a negative temperature coefficient of resistance, and the resistance varies with the temperature in a manner that can cause uneven heating or thermal runaway so that effective control is necessary. Other types of loads, such as silicon carbide heater elements, for example, have a resistance which changes with time. In such cases, it is usually necessary or desirable to maintain constant power input to the load, or to control the power input in a predetermined manner. This can be done by suitably adjusting the voltage applied to the load so as to control the current to maintain the desired power input even though the resistance, or other characteristics of the load, may change with temperature or time or in any other manner.
One way in which this type of control may be achieved is to provide a transformer capable of supplying the maximum required voltage and maximum current, with phase angle switching control by means of solid-state switch devices in the secondary circuit to adjust the effective output voltage. This, however, requires a very expensive transformer and is not an economical solution to the problem if the range of variation of the load resistance is more than about 2 to 1. A better approach is to provide taps on either the primary winding or the secondary winding of the supply transformer with switching means to change the tap connections for the desired voltage range as required by the resistance of the load. Phase angle switching control can be used in connection with taps on either the primary winding or secondary winding to obtain a smooth control of the output voltage from zero to a maximum value. Solidstate tap selector switches in the secondary circuit have also been used and, with phase angle switching control, smooth control of the voltage can be achieved. These arrangements have been widely used but they are relatively expensive because of the provision of taps on the transformer winding which requires a special design, so that the cost is high as compared to standard transformers. Furthermore, such a design is somewhat difficult to accomplish at the higher current levels that may be required, with a further increase in cost.